Fabrication of collimators employing optical fibers fusion-spliced to optical elements of substantially larger cross-sectional areas

ABSTRACT

A fiber collimator is provided, comprising at least two optical components, one of the optical components (e.g., an optical element such as a collimating lens or a plano-plano pellet) having a surface that has a comparatively larger cross-sectional area than the surface of the other optical component(s) (e.g., at least one optical fiber). The optical components are joined together by fusion-splicing, using a laser. A gradient in the index of refraction is provided in at least that portion of the surface of the optical element to which the optical fiber(s) is fusion-spliced or at the tip of the optical fiber. The gradient is either formed prior to or during the fusion-splicing. Back-reflection is minimized, pointing accuracy is improved, and power handling ability is increased.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of application Ser.No. 09/118,033, filed Jul. 17, 1998, now U.S. Pat. No. 6,033,515, issuedMar. 7, 2000, and is related (1) to application Ser. No. 09/450,471,filed Nov. 29, 1999, now U.S. Pat. No. 6,217,698, issued Apr. 17, 2001,which is also a continuation-in-part application of the '033application, and (2) to application Ser. No. 09/450,472, filed Nov. 29,1999, which is a divisional application of the '033 application.

TECHNICAL FIELD

The present invention relates generally to optoelectronics involvingoptical components of significantly different cross-sectional areas,such as optical fibers joined to optical elements such as lenses,filters, gratings, prisms, and the like, and, more particularly, tofiber collimators.

BACKGROUND ART

Fiber collimators find extensive use in optoelectronics, particularly inthe coupling of light from (to) an optical fiber to (from) a collimatinglens. Fiber collimators are basic components of telecommunicationproducts such as isolators, mechanical switches, couplers, circulators,optical switches, and wavelength division multiplexers. Such fibercollimators are fabricated by joining an optical fiber to an opticalelement.

While splicing of one optical fiber to another or of one optical fiberto an optical waveguide is known, the sizes are similar and localizedheating can be used to fusion-splice the optical components together.Splicing an optical fiber to a much larger optical element is morechallenging. For example, U.S. Pat. No. 4,737,006 entitled “OpticalFiber Termination Including Pure Silica Lens And Method Of Making Same”,issued to K. J. Warbrick on Apr. 12, 1988, discloses fusion-splicing anundoped (pure) silica rod to a single mode fiber to fabricate acollimator, employing an electric arc. However, this is an extremelycomplicated method and has limited applications.

The most often utilized processes for attaching optical fibers to thelarger optical elements include (1) bonding the fiber faces directly tothe optical element with adhesives or (2) engineering a complexmechanical housing which provides stable positioning of air-spacedfibers and optical elements throughout large changes in environmentalconditions.

The use of adhesives in the optical path of such devices is undesirabledue to the chance of degradation of the adhesive over time. On the otherhand, spacing the fibers a fixed distance away from the optical elementsby utilizing complex mechanical housings requires the use ofanti-reflection coatings at all air-glass interfaces in order tominimize losses of optical energy through the device. The presence ofair-glass interfaces also provides a source of back-reflected light intothe optical fibers. This phenomenon, known as back-reflection, is asource of noise in many communications networks, and effectively limitstransmission bandwidth of such communications networks.

The parent application to the present application provides a simpleprocess for fusion-splicing two optical components of different sizestogether, e.g., fusion-splicing an optical fiber to a much larger (atleast 2× diameter) optical element, using laser heating.

While butt-end coupling of the optical fiber to an optical element isdesired for simplicity, in fact, the prior art requires angle-cleavingof the optical fiber and angle-polishing of the optical element toreduce or minimize back-reflection. That is to say, the optical fiberand optical element are both processed to provide coupling at anon-perpendicular angle to the optic axis, which is parallel to theoptical fiber. Back-reflection resulting from simple butt-end couplingreduces the optical output and efficiency. In optical communicationsystems, back-reflection also has a detrimental impact on the BER (biterror rate) and the SNR (signal-to-noise ratio). Due to its uncontrolledgeneration and propagation, power reflected back in the fiber isconsidered excess noise when detected.

In previous art, it has been shown that positioning an angle-cleavedfiber or angle-polished fiber in proximity to the angle-polished face ofa collimating lens results in excellent collimation and excellentperformance characteristics of fiber collimators. However, theseexisting technologies for assembling collimators require very laborintensive active alignment techniques. The alignment techniques includemanipulating the position of the fiber relative to the lens in threelinear axes and three rotational axes during final assembly. If acollimator can be built that effectively makes the fiber and the lens asingle piece, then alignment can be reduced to two linear and tworotational axes during the fusion process and there is no need foralignment during final assembly, thereby reducing costs dramatically.

A key performance parameter to be minimized in collimator assemblies isback reflection of light down the fiber. By butt-coupling orfusion-splicing a fiber to a lens of the same refractive index, there isno apparent interface to cause back reflection. The beam is then allowedto diverge in the lens and does not see an index break surface until itexits the lens. By then, the beam is so large that the amount of lightthat can return to the fiber core is extremely small.

The laser fusion-splicing method disclosed and claimed in the parentapplication to the present application provides a back-reflection of −57dB. This may be acceptable for some applications. However, a furtherreduction in back-reflection would be desirable for other applications.

DISCLOSURE OF INVENTION

In accordance with the present invention, a fiber collimator isprovided, having reduced back-reflection, improved pointing accuracy,and improved power handling characteristics. The fiber collimatorcomprises at least one optical fiber fusion-spliced to an opticalelement, such as a collimator lens. The optical element is constructedfrom an optical material which has an index of refraction nearly equalto the index of refraction of the optical fiber to which it isfusion-spliced. For commercial reasons, pure fused silica glass ispreferred. In addition, the fiber collimator may comprise at least onefiber fusion-spliced to an optical element other than a collimator lens,such as a plano-plano “pellet” which is subsequently assembled jointlywith a separate collimator lens. This latter configuration is especiallyuseful in creating collimators with long optical path lengths andassociated large collimated beam diameters. The utilization of anoptical pellet provides all the advantages of reduced back-reflectionand improved power handling while reducing the required lens thicknessin long focal length collimators.

The splice created by the laser fusion-splice process will typicallyhave back-reflection of −57 dB. This slight residual back-reflection isdue to the small refractive index difference between the fiber core andpure fused silica. Even lower back-reflection can be achieved bycreating a thin axial gradient layer at the splice junction. Simpleadjustment of the fusion-splice process parameters is enough to favorthe creation of such an axial gradient through the diffusion of thedopant in the fiber core. The resulting back-reflection can be less than−65 dB with no detrimental effect on the quality of the splice. Similarresults can be obtained with a prior doping of a thin surface layer onthe optical element to be fused.

By attaching fibers directly to other optical components without usingepoxies or special termination techniques, costs are reduced,environmental stability is improved, alignment accuracy is enhanced,pointing accuracy is improved, and power handling is significantlyincreased.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand accompanying drawings, in which like reference designationsrepresent like features throughout the FIGS.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings referred to in this description should be understood as notbeing drawn to scale except if specifically noted.

FIG. 1a is a side elevational view of a prior art fiber collimator;

FIG. 1b is a side elevational view of one embodiment of the fibercollimator of the present invention;

FIG. 2 is a side elevational view, showing schematically the apparatusemployed in fusion-splicing one optical fiber to an optical element;

FIG. 3 is a view of an annular laser beam as it appears on the surfaceof a mirror through which the optical fiber is passed;

FIG. 4 is a side elevational view, similar to that of FIG. 2, showingthe fusion-splicing of two optical fibers to an optical element;

FIG. 5, on coordinates of back-reflection (in dB) and refractive index,is a plot of the calculated back-reflection from a fiber/pure-silicaoptical interface;

FIG. 6a is a side elevational, schematic diagram depicting an opticalfiber fusion-spliced to an optical element with a gradient layer in thejoining surface of the optical element;

FIG. 6b, on coordinates of refractive index and distance, is a plot ofthe refractive index as a function of distance along the optical fiber,through the gradient layer, and into the optical element;

FIG. 7, on coordinates of back-reflection (in dB) and thickness of thegradient layer, is a plot depicting the impact of an axial gradientlayer on the back-reflection of a fusion-splice;

FIG. 8, on coordinates of back-reflection (in dB) and power (in relativeunits), is a plot illustrating how a thin gradient layer can be createdand subsequently the back-reflection can be improved by adjusting thelaser power used for fusion splicing;

FIG. 9 is a cross-sectional view of a collimator of the presentinvention comprising a fiber fusion-spliced to a collimator lens; and

FIG. 10 is a cross-sectional view of a collimator of the presentinvention comprising a fiber fusion-spliced to a plano-plano opticalelement (“pellet”), assembled in combination with a separate collimatorlens.

BEST MODES FOR CARRYING OUT THE INVENTION

Reference is now made in detail to a specific embodiment of the presentinvention, which illustrates the best mode presently contemplated by theinventors for practicing the invention. Alternative embodiments are alsobriefly described as applicable.

A prior art fiber collimator 10 is depicted in FIG. 1a, comprising anoptical fiber optically connected to a lens 14. The optical fiber 12 ismaintained in position by a glass or ceramic ferrule 16. The lens 14 andferrule 16 with its optical fiber 12 are secured in a mounting sleeve18. A strain-relief elastomer 20 is provided at the end of the mountingsleeve 18 from which the optical fiber 12 emerges. The lens 14 andferrule 16 are provided with surfaces 14 a, 16 a, respectively, that areat a non-normal angle to the fiber 12. The fiber 12 is spaced from thelens 14 by an air gap 22.

For comparison, the fiber collimator 110 of the present invention isdepicted in FIG. 1b. It will be noted that there is essentially no airgap; the fiber 12 is coupled directly to the lens 14 by the fusionprocess described below. Further, there is no angle of the lens surface14 a with respect to the fiber 14; the lens surface is normal to thefiber. As a result, back-reflection is considerably reduced and pointingaccuracy is improved. Further, as a consequence of the process disclosedherein, power handling capability is considerably enhanced. Thereduction in back-reflection, improvement in pointing accuracy, andincrease in power handling capability are all discussed in greaterdetail below.

Localized heat has been effectively used in a variety of glassprocessing operations including surface polishing, fiber drawing, andfusion-splicing. The heat source used is frequently a simple resistanceheater or a controlled arc. All of the aforementioned processes can alsobe performed using a laser as a heat source.

Prior to the present invention, however, a method for splicing opticalcomponents of substantially different cross-sectional areas had not beendeveloped, to the knowledge of the inventors. The present invention isdirected to a method to form seamlessly fused monolithic pieces.

To fuse one or more optical components of a first cross-sectional areato an optical component of a substantially larger cross-sectional area,in one embodiment, the larger surface is first pre-heated by the laser.The pre-heat temperature is just sufficient to polish and melt thesurface of the larger component at the location one desires to fuse thesmaller component. Depending upon the size, it may be a heating of theentire surface or only a localized heating. The first surface(s) arethen brought into contact with the preheated surface and, once thethermal exchange is established (by conduction of heat), all componentsare heated simultaneously. If all surfaces are large (large with respectto the localized heating zone), then all may need preheating. Once thesurfaces are in contact at the appropriate elevated temperatures, fusionoccurs. The fusion temperature is just enough above the softeningtemperature to ensure a good flow of thermal energy between the twocomponents.

In a second embodiment, the fusion occurs starting with contact of allof the optical components and the components are never separated duringthe fusion-splicing.

In a third embodiment, all of the optical components are brought intocontact, then pulled back after alignment, and then fusion-spliced as inthe first embodiment.

Qualification of the interface is accomplished by measuring the backreflection of light through the system as well as mechanical testing.

There are no practical limitations in using this technique with respectto size mismatch, or the absence of a mismatch, or in cross-sectionalgeometry.

Any multiple pieces of optical elements, whether comprising an inorganicglass or an organic polymer, can be fused using the method of thepresent invention. The most common application will be fusion of singlemode fibers to optoelectronic or telecommunications devices.Fusion-splicing in accordance with the teachings herein virtuallyeliminates back-reflection and the associated losses. It is also verycost-effective, with a splice requiring a few seconds or less and theprocess can be fully automated. Splicing eliminates the need for activealignment in many instances. Splicing also ablates contaminants andprecludes the need for foreign materials, such as adhesives and otherorganic materials, in the optical path.

Optical inorganic glasses, such as silicas, silicates, borosilicates,borates, phosphates, aluminates, chalcogenides and chalco-halides,halides, etc., and optical organic polymers, such as acrylates,methacrylates, vinyl acetates, acrylonitriles, styrenes, etc., may bebeneficially employed in the practice of the present invention, althoughthe invention is not limited to the specific classes of materialslisted.

Because the heating is quick and localized, components can beanti-reflection-coated on surfaces other than the surface to be fusedprior to fusion. The process of the present invention also minimizes thenumber of coated surfaces. Typical assembly techniques leave a minimumnumber of surfaces to be coated: the face of each optical fiber beingspliced and both the input and output faces of the lens. However, theprocess of the present invention leaves as few as one surface becauseseveral surfaces (each optical fiber face and the lens input face) arecombined into a monolithic fused piece. Every surface, even when coated,contributes losses to the system because there is no perfectantireflection coating. Thus, reducing the number of surfaces to becoated reduces losses to the system. Furthermore, for conventionalcollimators, the coating used at the tip of the fiber considerablylimits the power handling ability of these elements. The method of thepresent invention eliminates the coating at the fiber tip and, in fact,eliminates the fiber-air junction all together. Collimators fabricatedin this fashion can handle considerably more power (>10×) than othertypes of collimators.

Pointing accuracy and beam quality can be monitored prior to fusion andlocked in due to fusion. Conventional (prior art) collimators have anintrinsic pointing error (the beam coming out of the collimatorpropagates along a line which is at an angle of approximately 0.5° withthe axis of the collimator). This error arises from the angle-polishedsurfaces and the associated air gap. In the present invention, whenconfigured with only one fiber, the collimator exhibits symmetry aboutthe axis defined by the fiber, and the pointing error can be reduced tovalues smaller than 0.1°. Such a small pointing error is a considerableadvantage for subsequent use of these collimators in devices because theoptical alignment becomes faster and simpler.

Elimination of angled surface index breaks reduces polarization effectssuch as polarization-dependent losses (PDL) and polarization modedispersion (PMD) in fabricated components. Current methods employoptical surfaces which are angled relative to the optical axis in orderto control back reflection, thereby inducing PDL and PMD above thoseinherent in the materials.

Another distinct advantage of the present invention is the thermalstability of the system. Because the parts are seamlessly fused into amonolithic piece, there is no dependence on the housing for maintainingsub-micron spacing tolerances as there is with other prior artapproaches in optoelectronic and telecommunications devices.

The present invention makes possible a very high quality and low costproduct for the optoelectronics/telecommunications industry. Withoutthis technology, one would be forced to use the prior art techniquesknown in the telecommunications industry, which are very costly, cannotperform as well, and/or use undesirable materials in the optical path.

The method of the present invention for splicing one or more smallcross-sectional area optical fibers to a larger cross-sectional areaoptical element comprises:

1. aligning the optical fiber(s) and the optical element along an axis;

2. turning on a directional laser heat source (such as an infraredlaser) to form a laser beam;

3. directing the laser beam to be collinear with the fiber(s) (this way,most of the laser light is not absorbed by the small fiber(s) but isreflected off surface because the reflection coefficient is very high atgrazing incidence);

4. ensuring that the laser beam strikes the larger cross-sectional areaoptical element at normal or near normal incidence so that absorption ofthe laser is much more efficient on the larger surface;

5. adjusting the laser power level to reach a temperature equal to orhigher than the softening temperature on the surface of the element toachieve fusion-splicing (and simultaneously achieve polishing andcontamination ablation); and

6. turning off the laser.

In the first embodiment, the two components (e.g., optical fiber(s) andoptical element) are aligned but separated by a space (typically a fewmillimeters), the laser beam is turned on to form the softening region,and the surface of the optical fiber(s) is brought in contact with thesoftening region of the optical element, the contact resulting in heattransfer to the surface of the optical fiber(s), which then softens,thereby achieving the fusion-splicing.

In the second embodiment, the two components (e.g., optical fiber(s) andoptical element) are first brought into contact and the laser beam isthen turned on to form the softening region where the two components arein contact to achieve the fusion-splicing.

In the third embodiment, the two components (e.g., optical fiber(s) andoptical element) are aligned, then brought into contact, then separatedby a space (typically a few millimeters), the laser beam is turned on toform the softening region, and the surface of the optical fiber(s) isbrought in contact with the softening region of the optical element, thecontact resulting in heat transfer to the surface of the optical fibers,which then softens, thereby achieving the fusion-splicing.

For fusion-splicing typical inorganic glasses, such as silica, a CO₂laser, which operates in the range 9 to 11 μm, is preferred, sincesilica-based glasses have very large absorption coefficient. Otheroptical materials typically have a large absorption in the infrared, andaccordingly, lasers operating in another region of the IR spectrum maybe used with such other optical materials.

The laser beam is collinear and grazes the optical fiber(s). This can beaccomplished in many ways. For example, a 45-degree mirror with acentral hole is used to redirect the laser beam collinear with the axisof the fiber(s) (the fiber(s) pass through the hole, parallel to eachother). Other methods that direct the laser beam along the axis of thefiber(s) may also be employed; such methods are well-known to thoseskilled in this art. The laser beam itself can be (but not necessarily)annular in shape. This last requirement is accomplished by differenttechniques: scanning system, special optical components (axicon), TEM₀₁laser mode, central obstruction, diffractive optical element, etc. Thesame effect could be accomplished by using two or more laser beams, allcollinear with the optical fiber(s).

The optical components being fusion-spliced preferably have similarthermal and/or mechanical properties. However, this is not a necessaryrequirement, since dissimilar optical components can be fusion-splicedemploying the teachings of the present invention. In such cases, thepossibility of strain due to the process may cause the splice to breakif the conditions are not right, and thus must be taken into account.However, such a consideration is well within the experience of theperson skilled in this art, and no undue experimentation is required.

FIG. 2 depicts the laser beam 24 impinging on the mirror 26, which has ahole 26 a therethrough. One optical fiber 12 passes through the hole 26a in the mirror 26 and is fusion-spliced to the optical element, e.g.,lens, 14. FIG. 2 depicts the optical fiber 12 just prior tofusion-splicing to the lens 14. FIG. 3 depicts an annular laser beam 24a in cross-section, along with the fiber 12. The optical element 14 maybe a lens, filter, grating, prism, WDM device, or other such opticalcomponent to which it is desired to secure the optical fiber 12. FIG. 4is a similar view depicting the fusion of two optical fibers 12 a, 12 bto the optical element 14. It will be appreciated that even more opticalfibers may be fusion-spliced to the optical element 14, employing theteachings herein.

The technology disclosed herein can be applied to conventional fibercollimators, expanded beam Collimators, WDM products, and any otherdevice that has a glass or polymer attachment site. One is no longerlimited to fusing components that only have substantially similardiameters.

As mentioned above, a key performance parameter to be minimized incollimator assemblies is back-reflection of light back down the fiber12. By butt-coupling or fusion-splicing a fiber to a lens of the samerefractive index, there is no apparent interface to causeback-reflection. In fusion-splicing a fiber to an optical element madeof pure silica, the difference in refractive index of the two elementsis so small that it is often neglected. However, this difference is notexactly zero because the core of the fiber has a slightly higher indexthan the bulk material. In fact, this difference in index is the basisfor the guiding properties of the fiber. For typical single-mode stepindex fiber this difference is approximately 0.36% (from the productinformation sheet of the Corning® SMF-28TM CPC6 Single-mode OpticalFiber) and will cause a back-reflection from the interface of −57 dB(=10 log[Power reflected/Incident Power]). For most applications, thissmall amount of back-reflected power is negligible. But for some cases,it is detrimental. In those cases, a back-reflection of −65 dB isconsidered an acceptable performance.

FIG. 5 illustrates the expected back-reflection from a fusion-splicedjunction as a function of the refractive index of the optical elementassuming the fiber is a single-mode step index fiber. The curves wouldbe very similar for all other types of fibers.

The refractive index of fused silica at 1.55 μis 1.444. Using a materialwith a slightly higher index would bring the back-reflection down from−57 dB to −65 dB. However, silica remains the best material because itsthermal properties are well-matched to those of the fiber. A bettersolution would be to create an intermediate layer 28 between the fiber12 and the optical element 14. The refractive index of that intermediatelayer 28 should vary gradually as illustrated in FIG. 6a and 6 b. Theexact profile of the index function is not critical. The intermediatelayer 28 is formed in the surface 14 a of the optical element 14, andextends a short distance D thereinto. The intermediate layer 28 mayinstead comprise a region 28′, shown in phantom, that is formed in theimmediate vicinity of the fusion-spliced fiber 12.

Such a gradient reduces the back-reflection of the interface. For alinear gradient of thickness D, the calculated back-reflection isillustrated in FIG. 7.

It can be seen from FIG. 7 that even a very thin gradient layer (<2 μm)considerably improves the back-reflection of the fusion-splice. Becausethe index profile and the thickness are non-critical parameters, thegradient layer 26 can be easily incorporated in the fusion-spliceprocess. To be effective, the gradient needs to be at least 0.2 μm inthickness. The thicker the gradient, the better the result. However,above a thickness of 2 μm, the incremental reduction in back-reflectionis negligible.

Different approaches to forming the gradient layer 28 are possible. Thefirst and most straightforward technique is to promote diffusion of thefiber core dopant(s) during the fusion-splice process. In the particularcase of single mode step index fiber 12, the core of the fiber is dopedwith a small concentration of germanium (Ge) to increase the refractiveindex. During the fusion-splicing process, a thin layer of glass at thetip of the fiber reaches a very high temperature and melts, allowing theGe-doped glass to spread and the Ge to diffuse. Various combinations ofprocess parameters (laser power, exposure time, junction pressure,post-heating, etc.) can be used to induce this thin gradient layer 28.For example, FIG. 8 illustrates the impact of laser power on themeasured back-reflection.

In this particular case, a fusion-splice is obtained by placing thefiber tip in contact with the back surface of the lens and by applyinglaser power in the fashion described in the present application. At thelow end of the laser power scale, enough heat is generated to melt thesurface and/or fiber tip, and perfect optical contact between the fiberand the back surface of the lens is achieved. This results in a backreflection on the order of −57 dB, consistent with the differencebetween the refractive index of the fiber core and fused silica. As thepower level is increased, there comes a point where the germanium in thecore of the fiber diffuses or separates in a thin layer at the junction.This layer acts as a refractive index gradient, reducing the backreflection to values below −65 dB.

The same overall results can be obtained by creating a thin axialgradient layer 28 on the surface 14 a of the optical element 14 prior tofusion-splicing. The obvious approach is to diffuse Ge, since this isthe dopant used to increase the index of the fiber core. One possibletechnique is to deposit a solution of germanium oxide on the surface anduse a CO₂ laser to incorporate the Ge within a thin surface layer ofmelted glass, Other in-diffusion, ion-implantation or ion migrationtechniques are also possible.

Thus, it will be appreciated that the gradient layer 28 is formed in atleast a portion of the surface 14 a of the optical element 14, eitherdirectly associated with the optical fiber(s) 12 and, possibly, theimmediate vicinity, or across the entire surface.

The fiber collimators prepared in accordance with the teachings of thepresent invention have high power handling ability. Power levels inexcess of 9 Watts have been reached without any indication of damage.This represents a factor of ten increase over the power of prior artfiber collimators, such as that depicted in FIG. 1a.

Fiber collimators produced using prior art methods are typically limitedto less than 1 watt continuous laser power throughput. Most havespecified power handling limits of 300 milliwatts. The failure mechanismof prior art collimators is typically catastrophic damage induced by thehigh laser intensity on the anti-reflection coated fiber facet. Due tothe very small central guiding region of single mode optical fibers (10micrometers in diameter is typical), very high intensity levels arereached (approximately 1 megawatt/cm²) at 1 watt throughput powerlevels. It is well-known that this intensity level often inducescatastrophic failure at optical surfaces.

However, by fusing the fiber directing to an optical element, such as apure silica collimator lens or plano-plano pellet, the fiber facet hasbeen effectively eliminated. The air-glass interface has been moved tothe output face of the attached optical element, where it has expandedto a typical diameter of 500 micrometers. The intensity at 1 watt laserpower throughput is only 500 watts/cm² at the output facet of theoptical element, representing a decrease in intensity of 2,500 times.Therefore, the power handling capacity of collimators produced inaccordance with the teachings of the present invention is many timeshigher than that of prior art collimators. Potential applications existin the areas of laser transmitters, high pump power fiber amplifiers,fiber lasers, laser projection displays, laser velicometers, medicallaser delivery systems, and industrial laser delivery systems.

FIGS. 9 and 10 are each a cross-sectional view of two preferredembodiments of the fiber collimator of the present invention. In FIG. 9,the fiber collimator 110 comprises the fiber 12 fusion-spliced to thelens 14, both contained in the mounting sleeve 18. A glass stabilizertube 30 supports the fiber 12 and the elastomer 20 provides strainrelief. As an example, the mounting sleeve 18 comprises 300 seriesstainless steel, provided with a 3 micron thick Cu—Ni—Au plating.Alternatively, the lens 14 and the glass stabilizer tube 30 may beprovided with a metal coating in place of the mounting sleeve 18. Thetypical diameter of the collimated beam is on the order of 0.5millimeters.

In FIG. 10, the fiber collimator 210 comprises the fiber 12fusion-spliced to the optical element 14, here a plano-plano pellet,both contained in the mounting sleeve 18. The mounting sleeve 18 issecured in a housing 32 which includes a chamber 34. The mounting sleeve18 protrudes into one end of the chamber 34 a distance. At the oppositeend of the chamber 34 is a separate collimator lens 114. The plano-planopellet 14 comprises a glass rod provided with plano-plano entrance andexit surfaces. A beam 36 emerging from the fiber 12 diverges in thepellet 14, which serves to minimize back-reflection; the pellet does notcollimate the beam. Instead, the diverging beam 36 is collimated by theseparate collimator lens 114. This configuration permits a much largercollimated beam, anywhere from 1 to 80 mm, and even larger. Such a largecollimated beam, without the configuration depicted in FIG. 10, wouldordinarily require considerably more glass. Thus, the configurationdepicted in FIG. 10 saves glass and its concomitant weight. Of course,the beam 36 could be incident on the separate collimator lens 114 andconverge onto the optical fiber 12.

INDUSTRIAL APPLICABILITY

The method of the invention is expected to find use in the fabricationof fiber collimators, employing fusion-splicing of at least one opticalfiber to an optical element having a relatively larger cross-sectionalarea.

Thus, there has been disclosed a fiber collimator, comprising at leastone optical fiber fusion-spliced to an optical element, such as acollimator lens, which includes a gradient in refractive index in thejoining surface thereof, and a method for the fabrication thereof. Itwill be readily apparent to those skilled in this art that variouschanges and modifications of an obvious nature may be made, and all suchchanges and modifications are considered to fall within the scope of thepresent invention, as defined by the appended claims.

What is claimed is:
 1. A fiber collimator comprising an optical elementand at least one optical fiber fusion-spliced to a surface thereof, saidsurface of said optical element provided with a gradient in index ofrefraction at least where said at least one optical fiber isfusion-spliced thereto, wherein said gradient in said surface of saidoptical element has a thickness of at least 0.2 μm and less than about 2μm.
 2. The fiber collimator of claim 1 wherein said optical element is acollimating lens having a surface to which said at least one opticalfiber is attached that is normal to said at least one optical fiber. 3.The fiber collimator of claim 2 wherein said collimating lens and saidat least one optical fiber are secured in a mounting sleeve.
 4. Thefiber collimator of claim 1 wherein said optical element is aplano-plano pellet having a surface to which said at least one opticalfiber is attached that is normal to said at least one optical fiber. 5.The fiber collimator of claim 4 wherein said plano-plano pellet and saidat least one optical fiber are secured in a mounting sleeve, saidmounting sleeve in turn secured in a housing provided with a chamber,said mounting sleeve located at one end of said chamber and acollimating lens located at an opposite end of said chamber.